Elevated levels of immunoglobulin associated with human platelets are believed to represent specific anti-platelet antibodies in patients with idiopathic thrombocytopenic purpura (ITP) by many investigators currently studying this condition. Because platelet destruction is markedly elevated in this disease and the plasma from patients with ITP can result in platelet destruction when transfused into normal recipients, ITP is believed to be an autoimmune disease. While evidence points to ITP being an immunologic disorder, the antigen(s) involved is obscure. While PAIgG is often elevated in ITP, the nature of this PAIgG is confusion. The apparent presence on the platelet surface of an Fc receptor make it difficult to distinguish between immune complexes which bind to these receptors and autoantibodies which bind to surface antigens. In addition, at least three classes of immunoglobulin have been shown to bind to various cell types while platelets are believed by most to bind only immunoglobulin G. Several studies have indicated that the binding of IgG to the platelet surface requires the Fc portion of the molecule. The binding of immunoglobulin and immuno complexes by an Fc receptor on platelets is of interest because it may result in an increased destruction of platelets by the reticulo-endothielial system.